Foam-dielectric coaxial cable with temperature-independent relative conductor length



March 16, 1965 R. P. LAMONS 3,173,990

FOAM-DIELECTRIC COAXIAL CA WITH TEMPERATURE-INDEPENDENT RELATIVE C D 0R LENGTH Filed Aug. 1962 Corn/H555 uucanmssssp 76 45 INVENTOR United States Patent Oflice 3,173,990 Patented Mar. 16, 1965 3,173,990 FOAM-DIELECTRIC COAXIAL CABLE WITH TEMPERATURE-INDEPENDENT RELATIVE CONDUCTOR LENGTH Robert P. Lamons, Chicago, Ill., assignor to Andrew Corporation, Orland Park, Ill., a corporation of Illinois Filed Aug. 27, 1962, Ser. No. 219,402 Claims. (Cl. 174102) This invention relates to coaxial cable, and more specifically to coaxial cable of the type using a foamed dielectric in the annulus between the inner and outer conductors.

Coaxial cables for radio-frequency use have in the past largely been confined, as regards commercial manufacture, to the two classifications generally known as solid dielectric and air dielectric, the former being typified by cables using polyethylene in solid form, and the latter by the type of cable in which the center conductor is supported by insulators occupying an extremely small portion of the total internal volume, normally at discrete intervals in large size rigid line and, in recent years, in the form of a spirally wound insulator such as a strip or tape of polyethylene or similar dielectric material occupying, at any given transverse section, a substantially negligible portion of the dielectric area. The solid cable is in most common use for less exacting applications, where losses in such materials are tolerable either because of relatively low frequency of operation or because particular applications permit high losses in order to obtain the advantages of flexibility and relatively low cost of manufacture as compared with airdielectric. At higher frequencies, particularly where power-loss is important, air-dielectric coaxial line or cable is used virtually universally, despite the fact that it is not only extremely expensive but also normally requires perodic flushing with dry air or gas, if not continuous pressure, in order to avoid the serious effects of moisture on the operation of such a cable or line.

At certain frequencies and for certain types of operations, the requirements imposed upon the dielectric as regards dielectric constant and losses lie essentially between the capabilities of solid-dielectric cable and airdielectric cable. (It will be understood that the term cable and line as hereinafter used refer to construetions having at least some degree of mechanical flexibility, being capable of being wound on some form of reel, etc., and confronted at least to some degree to the shaping requirements of a particular installation by bending and similar techniques, without the necessity of the plumbing-like angle bends and similar fittings required for rigid coaxial line, such as the type using tuned stubs for support of the central conductor.)

It has long been known that foamed plastics, such as foamed polyethylene, represent, from a theoretical standpoint, the ideal solution to the problem of provision of a type of cable which meets the requirements which cannot be met by solid-dielectric cable without the great expense normally associated with air-dielectric cable, particularly in view of the problems of maintaining the air or other gas filling of the latter dry. Furthermore, even in many cases where solid dielectric cable is acceptable as regards performance, the replacement of a large volume of the plastic by gas which is inherent in foamed plastics can well result in great economics, as has long been known. Accordingly, for many years there have been attempts to design fully satisfactory cable structures employing foamed plastics, particularly foamed polyethylene, as the dielectric, but all such attempts have previously had serious shortcomings which have prevented their widespread use.

At first glance, it would appear that the mere substitution of foam plastic in some existing structure using solid-dielectric or air-dielectric should easily produce the desired intermediate result as regards losses, at a cost much lower than that of air-dielectric cables. Attempts along these lines have, however, heretofore prevented the use in large volume of such structures as have been devised, since it was found that prior to the present in- Vention the only structures which gave fully satisfactory performance were incapable of being produced at costs greatly less than those of air-dielectric cables, so that the differential in cost justified the use of the foam type of construction only on a very limited basis in applications where it was clearly that an extremely small saving of money, plus the avoidance of air drying, would justify substantial defects or disadvantages in other respects, even though the low dielectric constant properties of the more expensive cable could be disregarded in a much wider area of the market.

Understanding of the difficulties heretofore encountered in utilizing the known theoretical advantages of foam dielectric can most readily be obtained by examining the problems introduced by the simple approach of substituting foam dielectric in cables employing other types of dielectric.

Considering first the making of such a substitution in the type of cable commonly used with a solid-dielectric, it would at first appear that completely satisfactory results should be obtainable. Foam polyethylene can readily be extruded on any size of center conductor wire or tube in a manner similar to that employed with soliddielectric, and has a similar, or somewhat greater, flexibility. Its lower material use (and lower dielectric constant) are produced by the existence of mutually sealed air bubbles or pockets throughout the foamed structure, and its general properties would accordingly seem to qualify it for the same use as that of the solid polyethylene, producing highly flexible cables of lesser dielectric constant, and accordingly less loss, and also at a somewhat lower cost. However, it was discovered early in the attempts to make use substitution that there were introduced problems making such a utilization of foamed plastic dielectric completely unacceptable. The great flexibility of the solid-dielectric cable is due primarily to the type of outer conductor which it uses, normally braided metal in tubular form fitting loosely on the outer surface of the solid polyethylene. Since the latter is extruded directly on the center conductor, there arises no problem whatever of moisture penetration into the dielectric, and such a cable is accordingly substantially impervious to most conditions of weathering, etc., the jacket normally used, of vinyl or similar plastic, being primarily to protect the braided outer conductor from scuffing and similar damage. The properties of the foamed material, however, are entirely different in this respect. In the first place, there is a slow but finite diffusion rate of moisture through the foamed plastic, so that prolonged exposure to rain and similar conditions, or immersion in water, makes such a cable completely useless. Even the device of adding a completely moisture proof outer jacket, greatly adding to the cost, proves to be of little basic benefit, since the braided outer conductor presents a virtually unimpeded path for water and moist air, thus limiting the lifetime of the entire cable to the appearance of a leak in the jacket. It will of course be understood that such a weakness does not mean that no such cables have ever been commercially manufactured, a number of manufacturers in fact presently offering such a foamed cable, but with the use very limited and indeed miniscule as compared with the potential market, as will readily be understandable from the fact that laboratory test leads and similar uses of coaxial cable in which extreme weathering conditions constitute no problem constitute a negligible portion of the consumption of cable as opposed to communications systems, military uses, telemetering and all of the other transmission-line uses which require ability to withstand all kinds of weather without damage.

Likewise, the attempts to substitute foam dielectric in structures designed for air-dielectric have, prior to the present invention, produced little advantage over the airdielectric construction itself, in many cases producing problems of such a serious nature as to make the use of the foam an overall disadvantage, rather than an advantage. The simplest type of air-dielectric construction uses a conventional tubular outer conductor of copper, copper-coated steel, or aluminum, with the center conductor supported by a polyethylene spiral or helix. In order to make this structure flexible, while at the same time confining the air, the outer conductor tube is made as thin as possible consonant with the mechanical requirements of durability against blows, etc. As with any air-dielectric cable, an air seal is made at the connectors, and dry air is kept under pressure, this being another limitation on the thinness of the outer conductor, which necessarily has very limited flexibility. When foam is substituted in this type of structure, it is again found that for a combination of reasons the advantages over the airdielectric are so dubious as to make the utility from an economic standpoint extremely limited, i.e., to leave virtually untouched most of the source of the large need for a cable intermediate between solid-dielectric cable and air-dielectric cable as regards cost and simplicity of handling and durability, on the one hand, and low-dielectric-constant electrical advantages on the other. The inadequacies of such a construction can readily be seen when the problems are considered. If the foam core fits only loosely, there is left an unsealed annulus creating the same type of necessity for sealing at the ends, flushing with dry air or gas, and accordingly keeping under pressure, as exists with air-dielectric. On the other hand, if the core fits tightly in the tube, as by drawing down the tube after insertion of the core to the point of producing slight compression of the core to give a good enough seal to make moisture penetration along the length negligible, there is introduced a new problem, related to a drawback or difliculty encountered with air-dielectric cables which has not yet been mentioned.

An incidental property of dielectric materials which can be employed in cables is the accompaniment of the electrical insulating properties by similar heat insulation properties. Accordingly, when a cable is subjected to abrupt change in temperature, a substantial temperature differential between the inner and outer conductors is created. Depending upon the direction of temperature change, the outer conductor will either be elongated or shortened with respect to the inner. In long cable runs of hundreds of feet or more, this differential ex pansion or contraction becomes very sizeable, the development of a diiferential of several inches, or even a foot or more, being not uncommon. In the air-dielectric cable using such a plain tube, this differential is normally accommodated by designing the end connectors with a sliding joint between the center conductor and the outer shell of the connector to absorb such temperature differential effects. The necessity for this relatively complex type of connector is another factor of both cost and inconvenience of assembly, together with problems of noise generation at the sliding contact, etc., which have encouraged the use of the solid dielectric type wherever reasonably possible.

Considering now the effect of this thermal expansion differential on the construction in which foam is employed as the dielectric, the nature of the new problems introduced (in addition to failure to solve the problem of the necessity of the sliding-joint connector type mentioned) may be understood. The only economical method of manufacture involves the extrusion of the foam on the center conductor, this assembly later being surrounded by the tube. The inner surface of the foam sleeve is securely locked to the inner conductor along its entire length in the extrusion process. If the outer conductor is drawn down on the assembly tightly enough to make a resonably secure moisture seal in the annulus, the outer conductor tube now becomes locked securely to the outer surface of the foam in a generally similar manner. When such a construction is exposed to an abrupt and large temperature drop externally (or if the center conductor should become heated by the dissipation of power being propagated or otherwise), there is created a condition potentially destructive of the cable. Since thermal expansion forces in metal are so high as to be virtually infinite, this differential expansion must inevitably either shear the dielectric axially or fragment the outer conductor. When it is attempted to use thin outer conductors, both for the provision of flexibility and for reduction of cost, the outer conductor can, under suitable conditions, completely lose its unitary character, checking, cranking, and shattering into tiny fragments along its entire length, while even with somewhat thicker outer conductors, there will inevitably be produced suflicient cracking and appearance of fissures to destroy the wall moisture seal oven if adequate electrical properties as a single continuous conductor are sufficiently preserved. In theory, it is of course possible to employ constructions permitting slippage, such as by using a compound foam core with relatively slidable concentric portions, or polishing one or both of the metal surfaces, with or with out treatment of the corresponding surface of the foam, to permit such slippage at the interface. In practice, however, the permitting of such slippage while at the same time maintaining the moisture seal along the length is extremely difficult, if not impossible, in any economical production process. The selection or treatment of the foam to permit the expansion by stretching, etc., without breaking is essentially impossible when it is observed that at least half of the total differential must appear as a change of relative position at an end of the cable. Nor is it practical to select or treat the dielectric so that it will readily shear, since strength of the dielectric in all directions is a requisite to permit the bending and shaping of the cable for which the flexibility is desired without impairing the electrical properties. Accordingly, with the tube construction here being discussed, the only safe way of guarding against rupture of the outer conductor tube when it is drawn down sufiiciently tightly to make a good moisture seal is to make it sufficiently thick so that under the most adverse conditions which may be encountered as regards temperature differential, it will produce shearing of the dielectric before cracking of the outer conductor. Not only is the disadvantage of this solution to the expansion problem unsatisfactory from the standpoint of flexibility and of cost of the outer tube, but additionally, as will be observed, it presents no solution to the necessity of sliding joints in the connectors. Even if some way is found to so closely control the surface conditions of the dielectric and the outer conductor, and the exact degree of friction between them produced by the drawing down of the tube, so that slippage is assured without breaking the longitudinal moisture seal, the disadvantages inherent in the sliding connector joint would remain.

One method of construction for increasing the flexibility of coaxial cables, and also eliminating sliding joints in the connectors is the use of a corrugated outer conductor. Such a construction is shown in Brandes, et al., United States Patent 2,890,263, for example. When employed with air-dielectric cable, the corrugated construction, in addition to having the advantages described in that patent with respect to strength and flexibility,

permits the use of end-connectors in which the longitudinal positions of both the central and outer conductors are securely moored, the over-all length of the cable being fixed by the length of the center conductor (and varying with its temperature and resultant length), any differential being absorbed by the deformation of the corrugations produced by the pushing or pulling between the clamped ends of the outer conductor. Where this type of construction is employed with long cables, the connectors must be of great strength, with extremely rigid clamping of both the inner and outer conductors, since the appearance of any expansion differential which would, except for clamping, produce a difference in end positions, imposes on the connectors a mechanical load equivalent to that of pushing or pulling the end of the outer conductor to produce an equivalent compression or elongation.

The patent just mentioned, like other prior art discussed above, suggests the use of foam dielectric with such a helical corrugated construction, showing two ways in which this may be done, one in which the foam dielectrio is in the form of a smooth, round cylinder tangent to the roots of the corrugation of the outer conductor, and one in which the dielectric extends to complete filling of the crest. The present invention stems from experimentation with structures somewhat resembling those of the foam dielectric structures shown in the mentioned patent. it has been found that by some simple modifications of this general type of structure, there can be ob tained foam dielectric cables much more closely approaching solid dielectric cables in flexibilty and cost, and in the type of connectors which may be used, than any heretofore known, thus making available the electrical superiority of foam dielectric cables at a cost far below that of air-dielectric cables, and without the inconveniences and expensive accessories required for air-dielectric cables.

Basically, the structure of the present invention, in its more specific aspects, will best be understood by first examining the characteristics of cables employing foam dielectric in a corrugated outer conductor in the two manners just described. Taking first the employment of a completely round annular foam sleeve formed, as is normal, by extrusion on the center conductor, and of a diameter matching the inner or minor diameter of the corrugated outer conductor into which it is inserted, the resulting structure has, except for the superior characteristics imparted by the corrugation as regards flexibility and strength for any given wall thickness, properties very much like those of an air-dielectric cable except for the increased dielectric constant, the air leakage, both around the helical void or air-space created by the helical crest and through the loose joint or contact line between the root of the corrugation and the circumference of the dielectric sleeve, making the problem of moisture a severe one, thus continuing to require the provision of atmospheric seals, flushing of dry air or gas, etc., a necessity. Likewise, the end connectors must be of the special high-load type used with the air-dielectric. Such a construction is typical of those described above as having been found to possess so little advantage over air-dielectric cables, confined practically entirely to the small diminution in cost of manufacture, that they have found little use.

When the construction employing the complete filling of the corrugated outer conductor, other than the central portion occupied by the inner conductor, with the foam is examined, the characteristics are found to be substantially different. It is found that a completely satisfactory moisture seal is made, thus eliminating the necessity for any air or gas filling or circulation. However, it is also found that the flexibility of the cable has been badly impaired as cornpared with either the other type of foam construction just mentioned or as compared with the air-dielectric type, the reasons for these findings being fairly obvious. However, with particular constructions (meeting requirements later to be described), upon measurement of the force required at the ends of any given length of cable to hold the lengths of the inner and outer conductors constant under conditions creating differential expansion, it is found that the required force, i.e., required strength or mechanical load of the connectors, has been greatly reduced. When the ends of the cable are disconnected under temperature conditions producing differential expansion, the amount of the differential is found to be similarly greatly reduced. It is thus found that in addition to the elimination of the moisture problem, the combination of such a cable with end connectors of the anchor type also eliminates the elaborateness and expense associated with high-strain connectors of this type and with connectors of the sliding-contact type.

Although the cable-and-connector combination just mentioned represents a substantial advance in the quest for a fully satisfactory foam-dielectric cable, in the respects just mentioned, there remain, in this construction, substantial drawbacks preventing wide utilization. As already indicated, the flexibility is somewhat impaired as compared with an air-dielectric cable or the type of foam cable in which the foam extends only to the root of the corrugation. The ultimate objects of the present invention are accomplished in a manner which obtains, and in some cases increases, the advantages of these various constructions when considered individually, combining the best features of each to produce a foam-dielectric coaxial cable of a cost and convenience far more closely approaching that of solid-dielectric cable than anything previously known. The manner in which this has been accomplished will best be understood from a description of the construction of the embodiments of the invention illustrated in the annexed drawing, together with discussion of the theory of the operation as best presently known.

in the drawing:

FIGURE 1 is a view in elevation, partially broken away in longitudinal section, of a coaxial cable made in accordance with the invention;

FIGURE 2 is a transverse sectional view taken along the line 2-2 of FIGURE 1;

FIGURE 3 is a more or less schematic view illustrating, in exaggerated form, the deformation of the outer conductor of the cable of FIGURE 1 under conditions producing thermal expansion and contraction;

FIGURE 4 is a fragmentary sectional view of a cable of modified form incorporating the teachings of the invention;

FIGURE 5 is a fragmentary sectional view illustrating another modified form of cable within the teachings of the invention;

FIGURE 6 is a fragmentary sectional view illustrating the application of the broader teachings of the invention to a further variation of this aspect of the invention;

FIGURE 7 is a further fragmentary sectional view of a still further modification; and

FIGURE 8 is a more or less schematic view illustrating the cable of FIGURE 1 with end connectors of the simple type (diagrammatically illustrated) normally used with solid-dielectric cables.

The illustration of FIGURES l and 2 will be seen to show a cable appearing very similar, as regards features capable of ready illustration, to one shown in Brandes et al., US. Patent 2,890,263, the difierences which produce the present results lying in aspects of construction and choice of parameters to be later described. The cable therein illustrated, generally designated by the numeral 10, has an outer conductor 12 coaxial with an inner conductor 14, the entire interior of the outer conductor, other than the inner conductor 14, being filled with foam dielectric 16. The outer conductor 12 is helically corrugated, any given longitudinal section accordingly having roots 18 and crests 2t), alternately, in more or less sinusoidal fashion. In the present instance, however, there are substantial differences in compression or density of the dielectric in the various portions of the interior of the outer conductor. The dielectric in contact with the inner surface of the outer conductor at the roots 18, designated by the numeral 22, is under substantial compression, while the dielectric in the crests 24 is uncompressed, thus producing great differences in density of the dielectric and in mutual pressure between the surface of the dielectric and the outer conductor in these respective regions. These differences have two important results. First, although the crests are filled with the foam dielectric, the adverse effect on flexibility for any given average dielectric constant is substantially reduced. This may be understood by studying the forces which operate when the illustrated cable is flexed. It will be seen that the effect of the dielectric at the roots is much smaller than that in the crests in opposing bending. By incorporating the component of the average at the roots, and the lower density component in the crests, the effect on flexibility, for any given value of dielectric constant, is substantially reduced, this being in essence, to a considerable extent, the reason for the excellent flexibility characteristics with uniform cylindrical foam of smaller diameter, i.e., with the crests completely vacant. This discussion, as thus far stated, assumes, of course, that the same outer conductor is used in either case. Actually, however, the backing of the crests of the outer conductor by the uncompressed, but firm, foam, permits the use of a sufliciently thinner outer conductor, for any given degree of required strength, so that the over-all flexibility, particularly when considering lifetime of the outer conduct-or as limited by fatigue due to repeated flexing, is as good as, or better than, in the case where the crests are left void. The second effect of this differential in compression, and perhaps more important, is the creation of a tight mechanical coupling for longitudinal motion between the dielectric and the outer conductor in the region of the roots 18. This tight and slip-free coupling is produced by the combination of the high degree of friction and the interlock which is created at this point, the compression being tapered from a maxi mum at the roots to a minimum at the crests. With this construction, as with the tight coupling previously described in connection with a plain tube outer conductor, the roots are firmly locked to the corresponding points on the inner conductor radially directly inward thereof, so that their position is at all times dictated by the condition of elongation or contraction of the center conductor, the lower density or state of compression of the dielectric in the crests permitting compression of this portion in the bellows action thus produced in each adjacent pair of roots in accommodating to elongation and contraction of the center conductor. Thus the over-all effect is that each small segment itself provides its own force of compression or expansion of the corrugated outer conductor whenever conditions of diflerential expansion exist, and it is found with this construction the problem of differential expansion is eliminated for all significant purposes, tests showing no readily measurable over-all elongation of either conductor with respect to the other at the extreme ends of even a very long cable under conditions producing serious differential expansion effects at the ends with any other known construction. The absence of differential expansion permits the use of extremely simple end-connectors in all respects closely similar to those heretofore used only with cables of much higher dielectric constant.

Differential expansion between the inner and outer conductors may occur in a number of different ways. In FIGURE 3 there is shown the manner in which the present construction self-compensates for heating and cooling of the outer conductor in response to ambient conditions. As therein shown, the roots 18 are held rigidly in place by the dielectric (not shown in that figure), the crests 20 of the outer conductor 12 absorbing the expansion due to heating, as shown in dotted form at 12a, or the contrac tion due to cooling, as shown in dotted form at 121). It will of course be understood that the expansions and con tractions shown are grossly exaggerated in the more or less schematic view of the drawing, and it will also be understood that the drawing does not show the change of diameter and circumference which will of course also occur, being confined to a showing of the longitudinal expansion and contraction which is the important element in the present invention. As ambient conditions are reached on the interior due to the small, but finite, heat conductivity of the foam plastic and exposure (where occuring) of the ends of the cable to ambient conditions, the original position of the corrugations will gradually be restored by the increase or decrease of the spacing between the roots 18 occurring with the expansion or contraction of the center conductor, the net overall effect after a new position of equilibrium is reached being restoration of the original condition as shown in FIGURE 3, with a slight alteration of the scale of the drawing. Similarly, as indicated above, if the non-equilibrium condition is initiated by the center conductor, the spacing of the roots 18 follows the change to produce the same general conditions as illustrated by the dotted representations of FIGURES 12a and 12b. Where the outer conductor contains a wrapping or protective cover of substantial heat-insulating properties, the original equilibrium will be largely restored with the passage of time. However, under some conditions, such as high heat generation in the center conductor and large heat loss from the outer conductor, large differentials may continue to exist during periods of use of the cable.

In addition to the feasibility of using the simplest kind of coaxial connectors, the self-correction of the present cable for thermal effects offers great advantage in avoiding the necessity for assuring the existence of equal temperature conditions when cut-off of a piece of cable is made from a long length of cable, for example on a reel. Where cable is of the type which does not self-compensate for thermal effects, as the present cable does, it is a necessary precaution in making a cut-off to assure that the piece from which the cut-off is being made has been sufficiently seasoned in the existing ambient temperature to reach complete equilibrium in order to avoid cable waste in subsequent trimming of the inner or outer conductor, as the case may be, of both the piece cut off and of the remainder when temperature equilibrium is reached. Furthermore, even where such waste is tolerable, the avoidance of similar seasoning before installation of the connectors is highly advantageous. With the present construction, a reel may, for example, be brought indoors immediately from a severely cold storage place, cut off, and have the connectors installed thereon without any necessity for de lay, and similarly a reel may be brought outdoors from a warm place, the cut-off made, and the piece so cut off returned indoors for immediate assembly of the connectors. Such handling, heretofore generally obtainable only with solid dielectric cables, removes a further impediment heretofore existing to the use of foam dielectric.

The fabrication of the cable of FIGURES 1 and 2, including the desired production of the differential compression contributing to the results described (with benefits more easily understood from embodiments of a less complex nature from the standpoint of theory but of greater difliculty of manufacture, later to be described) may be performed in a very simple fashion. The assembly of the inner conductor surrounded by the fully adhered foam dielectric may be done in the conventional manner, the center conductor being roughened, if required, to produce secure adhesion, and the foam then being extruded onto the center conductor as a uniform round tubular sleeve. The outer conductor is then placed around the core, either by insertion of the core into a unitary tube or, preferably, by formation of the tube around the plastic core by the continuous feeding, bending, and welding of strip material, as previously known in connection with air dielectric cables The corrugations, and the compressed and relatively uncompressed regions of the dielectric sleeve, are then formed by a simple helical corrugation operation with a suitable blunt tool of the type commonly used for this purpose. A commercial embodiment employing the construction illustrated in FIGURES 1 and 2 uses a copper tube of mil thickness as the outer conductor, with a major outer diameter of .54 inch and corrugations of 30 mil depth, with a pitch of .2 inch and a copper inner conductor of .158 inch, the corrugating operation producing the required depth being performed on an outer conductor tube tightly fitting the core before the corrugation operation. This cable, employing a foam polyethylene of a dielectric constant of approximately 1.5, has an impedance of 50 ohms, a DC. breakdown voltage of far more than 4,000 volts and can be repeatedly bent and rebent on a 6-inch radius without cracks or other deleterious effects, and displays no readily measurable differential expansion of inner and outer conductors under fairly extreme test conditions Without confinement of the cable ends.

In FIGURE 4 is shown a variation of the construction of FIGURE 1. In this construction the outer conductor 26 again forms a highly compressed dielectric at the roots at 28, but the portion 30 of the dielectric extending into the region of the outer conductor, between the roots does not completely fill the crests, leaving voids or air spaces 32 therein. The outer conductor 26 may be so shaped that the bulk of the region between the crests remains unfilled by the dielectric. Such a structure has the advantage, in certain cases, of again further reducing the deformation force required to be exerted at the roots in order to deform the shape of the crest with any given thickness of the outer conductor, thus permitting the use of the construction, for example, with fine center conductors, or with center conductors formed of thin tubes, and also permitting higher degrees of foaming of the dielectric or other characteristics which might otherwise impair the ability of the dielectric to absorb the stress required to deform the corrugations in accordance with the temperature conditions as described above. Such a cable is made in a manner closely similar to that described above. A commercial embodiment corresponding substantially to that of FIGURE 4 uses a 10 mil copper tube outer conductor with a major outer diameter of .98 inch, corrugation depth of 60 mils, a corrugation pitch of .283 inch and an inner conductor of copper wire of .3125 inch outer diameter. In this case the tube is not tightly fitted over the foam sleeve before corrugation, thus leaving voids or gaps as shown at 32 in FIGURE 4. This cable again has a 50 ohm impedance, with a DC. breakdown voltage, of course, substantially higher than in the previous case (well in excess of 6,000 volts), with repeated bending on a 12- inch radius being accompanied by no damage, due to fatigue or otherwise, and with the absence of differential expansion appearing at the ends again being obtained. Both this and the previously described commercial cable use connectors which are in all respects closely similar to those used with braided-outer-conductor cables of comparable size, no substantial stress appearing between the inner and outer conductors.

There is shown in FIGURE 5 a further modification of the invention which is slightly more difficult to manufacture, but which is easily understood from the standpoint of the theory of operation, and serves to form a foundation for understanding of certain necessary aspects of the selection of materials, thicknesses, etc., and also certain of the advantages, of the embodiments already described. In the embodiment of FIGURE 5, the outer conductor 34 has its root portions substantially at the surface of the dielectric sleeve 16, which is in its original form as extruded, being uncompressed by the outer conductor. It will be seen that this construction as so far described is closely similar to a construction described in the patent previously mentioned. There are, however, important differences, in part shown in the drawing and in part incapable 10 of simple illustration. In the embodiment of the present invention shown in FIGURE 5, the region of the foam core in contact with the bottom or root of the corrugations is coupled to the root by fusion of the plastic at 36, this coupling being formed without substantial depressing or other deforming of the dielectric by the application of heat to the outer conductor, preferably simultaneously with the corrugation of the loosely fitting tube by which this construction is fabricated, although the same general effect can be obtained by inserting the assembly of core and inner conductor (the latter not shown in this view) in a corrugated tube already formed, with the heat being applied subsequently. When this embodiment of the broader teachings of the invention is considered, the manner of operation and advantages of the embodiments earlier described become in some respects more obvious, and also lead to understanding of the manner in which materials, thicknesses, etc., are selected for practice of the invention. As Will be more easily seen here than in the embodiments previously described, any tendency, resulting from thermal effects, toward increase of the spacing between adjacent coupling regions 36 joining the outer conductor to the dielectric will appear as tension on the portion of the dielectric extending between the points of coupling. If the overall rigidity, including tensile strength, of the plastic, is insufficient, in the particular thickness or radial extension of the dielectric, to deform the corrugation, the outer conductor and the inner conductor will not be kept in corresponding positions. The same criterion extends to the strength of the coupling at the roots. It will be seen that for any given thickness and material of the outer conductor, with other factors con stant, there can be reached a thickness of the insulator sleeve at which it becomes incapable of coupling the corresponding points of the inner and outer conductors. This condition is corrected by reducing the force required to deform the corrugation. Likewise, the resistance of the corrugations to deformation is related to the required tensile strength of the inner conductor. If the force required to deform the corrugations is higher than the breaking strength of the inner conductor (or strength against crushing by endwise force in the case of a tube), and the foam dielectric and its couplings with the inner and outer conductors meet the requirements mentioned, the resultant structure creates a condition which might be described as the inverse of that previously mentioned as a problem in the design of foam cable using plain tubes as outer conductors, i.e., differential expansion stresses may break the Weakest conductor. Accordingly, the utilization of the present invention requires that the material and thickness of the outer conductor, and the details of construction of the corrugation (that of FIGURE 1 versus the solid showing of FIGURE 4, versus the dotted showing of FIGURE 4, etc.) must be so interrelated with the strength and thickness of the dielectric and with the strength of the coupling between the dielectric and the re spective conductors, and with the strength of the inner conductor, that the desired correspondence between inner conductor portions and outer conductor portions will be preserved. Obviously, this requires that the resistance of the dielectric to differential longitudinal stretching between the inner and outer radial portions thereof and the shear strength of the dielectric and the strength of the longitudinal couplings at the coupling regions with the outer conductor all be greater than the resistance to deformation of the outer conductor at the coupling regions. Otherwise, as may readily be observed, the differential expansion will simply produce relative longitudinal stretching or shrinking of one radial portion with respect to the other, or the dielectric may shear, or the outer surface may break or slide away from the attachment to the outer conductor at the coupling regions, any of these preventing the achievement of the avoidance of substantial differences of relative positioning of the ends of the inner and outer conductors due to thermal effects.

ill.

Fortunately, the properties of foam dielectric such as foam polyethylene are such that these criteria can readily be met for most or all cable constructions by simple experimentation with design variations required for particular purposes. Solid copper inner conductors and outer conductors of the same material make the design of cables using the invention extremely simple, particularly in cables of relatively small size. With these materials, any of the corrugated structures shown can readily be made, using copper of the order of mils thickness, over a large range of cable sizes and impedances. For certain purposes, it is sometimes desirable to design cables with outer conductors of materials of much greater rigidity, such as steel coated with copper, as described in the Brandes patent. In such a case, the design becomes more critical, and the corrugations must be carefully dimensioned, and the manner of coupling the dielectric to the outer conductor (i.e. the filling of the crests of the corrugations) carefully controlled to minimize the required force for deforming the corrugations.

In FIGURE 6 is shown a somewhat different construction for the outer conductor, illustrative of the broader teachings of the invention, and designed for use where the excellent mechanical properties of the corrugated structure are secondary, but the outer conductor is to serve as a moisture seal for the foam dielectric. Here the outer conductor is joined to the dielectric at coupling points or regions 42 by adhesive or by fusion of the dielectric in a manner similar to that discussed in connection with FIGURE 5, the regions 44 of the outer conductor intermediate between the coupling points to the dielectric being essentially limp and formless, but being, as in the case of the corrugations, of greater longitudinal length than the distance between the coupling points to accommodate expansion and contraction of the dielectric in accordance with the dimensions of the inner conductor. Such a construction will be seen to utilize the invention slightly differently than the embodiments previously described, since a limp or slack outer conductor, such as a foil, cannot in any event exercise strain on the end connectors. It is found, however, that the advantages of this construction over constructions in which a very thin tube, foil or foil-like, is simply made of extra length, with slack or excess used as a single overall differential expansion compensation, the end being moored to the center conductor at the connectors, remains substantial. Where the foil (or a thin tube) is not coupled to the dielectric at very small spaced intervals, the possibility of tearing of the structure upon occurrence of a large overall expansion differential is relatively high as compared with the present construction. In the present construction, even the most extreme of conditions in the overall length of a cable produces only the most minuscule motion in the slack portions 44, so that such a construction can easily be placed under relatively tightly fitting jackets, etc., normally used for the protection or" such a thin conductor. This fact also permits the construction of FIGURE 6 to be used with foils and thin tubes having open lap seams, the moisture protection being provided by moisture-impervious fairly tight jacketing.

FIGURE 7 shows still another embodiment of the invention, which might be considered as having a conductor of semi-corrugated construction. Here the outer conductor 4-6 is of a resilient material such as a Phosphor bronze of high conductivity, internally coated with copper if so desired, and the coupling to the dielectric is made by the fitting of V-shaped points formed on the outer conductor 45 into corresponding notches in the insulator, preferably by the employment of an appropriately pointed scoring or corrugating tool applied with sufficient force to permanently deform the outer conductor 46 and the plastic foam as at 48, the intermediate regions 56 again being longer than the actual spacing between the coupling points, the confinement of hese portions by a jacket (not shown) serving to keep the coupling joint intact.

There is shown in schematic form in FIGURE 8 a cable assembly in which a cable made in accordance with the invention (the cable of FIGURE 1 being illustrated) is provided with end connectors 52, these being of a completely conventional type having an outer shell 54 mechanically and electrically connected to the outer condoctor of the cable and a center conductor 56 receiving the inner conductor of the cable. The ends of the crest may be plugged with grease in using the cable of FIG- URE 4 or 5.

It will of course be understood that a factor which has not been mentioned herein relates to the expansion and contraction of the foam dielectric itself. Although the coefficient of expansion of this type of material may be substantially higher than that of a metal, the forces which are exerted in the temperature expansion and contraction of this type of material are not of the same magnitude as those of the relatively infinite force exerted by the expansion and contraction of the metal parts of the cable. Furthermore, it will be observed that in the problem of differential expansion and contraction in the longitudinal direction of the inner and outer conductors, the dielectric has an intermediate average temperature. The lack of large effect of the dielectric expansion and contraction is particularly marked in the case of transient effects, which are usually of greatest magnitude before there has been substantial penetration of the plastic by the abrupt temperature change. However, the fact that the dielectric itself is expanding and contracting inevitably produces some effect, which must be borne in mind in the design of utilizations of the invention which may be marginal as regards the basic critical factors mentioned above.

Although a number of embodiments of the invention are herein described, persons skilled in the art will readily see that these are merely illustrative, and that a large number of variants, differing greatly in appearance and details, can be devised which nevertheless use the basic teachings of the invention. Accordingly, the invention should not be considered, as regards the scope of the patent protection to be afforded it, as being limited by the particular embodiments herein shown and described, but shall be determined in accordance with the structures as described in the annexed claims, and equivalents thereof.

What is claimed is:

1. In a coaxial cable comprising an inner conductor, a generally tubular outer conductor coaxially surrounding the inner conductor in spaced relation, and a foam dielectric rigidly secured to the inner conductor in the space between the conductors, the improved construction characterized by (a) longitudinally spaced coupling regions between the dielectric and the inner surface of the outer conductor,

(b) the length of the outer conductor between said coupling regions being greater than the longitudinal distance therebetween,

(c) the coupling between the dielectric and the inner surface of the outer conductor for relative motion in the longitudinal direction being substantially smaller between the coupling regions than at the coupling regions,

(:1) the resistance of the dielectric to differential longitudinal stretching between the inner and outer radial portions thereof and the shear strength of the dielectric and the strength of the longitudinal couplings at said coupling regions all being greater than the resistance to deformation of the outer conductor in response to relative longitudinal forces applied at the coupling regions, and

(e) so that the coupling regions remain in the same relative longitudinal positions with respect to all portions of the length of the inner conductor des te differentials in thermal expansion and contraction of the conductors.

2. The improved cable of claim 1 having as the outer conductor thereof a unitary sealed tube having corrugations, the coupling regions being at the roots of the corrugations.

3. The improved cable of claim 2 wherein the coupling regions are formed by inward compression of the dielectric by the roots of the corrugations.

4. The improved cable of claim 3 wherein the dielectric extends radially outward of the coupling regions in the longitudinal spaces therebetween, all dielectric so extending being compressed less than the portion in the coupling regions.

5. The improved cable of claim 4 wherein the crests of the corrugations are substantially free of dielectric.

6. A coaxial cable assembly comprising a cable having an inner conductor, a corrugated outer conductor coaxially surrounding the inner conductor, a foam dielectric in the region between the conductors locked to the inner conductor along substantially its entire length and coupled to the outer conductor for longitudinal motion at least at the root of each corrugation, the resistance of the dielectric to differential longitudinal stretching between the inner and outer radial portions thereof and the shear strength of the dielectric and the strength of the longitudinal couplings at said coupling regions all being greater than the resistance to deformation of the outer conductor in response to relative longitudinal forces applied at the coupling region, and co-axial connectors at the ends of the cable having inner and outer conductors fixedly attached to the inner and outer conductors of the cable, all differential expansion forces being applied to the outer conductor corrugations by the dielectric and the connectors being substantially free of differential expansion stress.

7. A coaxial cable comprising an inner conductor, a corrugated outer conductor coaxially surrounding the inner conductor, a foam dielectric in the region between the conductors locked to the inner conductor along substantially its entire length and coupled to the outer conductor at least at the roots of the corrugations, the coupling strengths of the couplings between the dielectric and the respective conductors and the tensile strength of the dielectric and the inner conductor and the resistance of the dielectric to differential stretching of the inner and outer portions being greater than the longitudinal deformation resistance of the outer conductor, so that the conductors remain of equal length despite differential ex- 5 pansion.

8. A coaxial cable comprising an inner conductor, an outer conductor coaxially surrounding the inner conductor, a continuous foam dielectric in the region between the conductors locked to the inner conductor along substantially the entire length of the dielectric and coupled to the outer conductor at longitudinally spaced coupling regions, the portions of the outer conductor between said regions being longer than the longitudinal spacing, the coupling strengths of the couplings between the dielectric and the respective conductors and the tensile strength of the dielectric and the inner conductor and the resistance of the dielectric to differential longitudinal stretching between the inner and outer radial portions thereof and the shear strength of the dielectric being greater than the longitudinal deformation resistance of the outer conductor in response to relative longitudinal forces applied between coupling regions, so that the coupling regions remain in the same relative longitudinal positions with respect to the inner conductor despite differences in thermal expansion and contraction of the conductors.

9. The coaxial cable of claim 8 wherein the dielectric is fused to the outer conductor at the coupling regions.

10. The coaxial cable of claim 8 wherein the outer conductor is helically corrugated and the coupling regions are at the roots of the corrugations.

References Cited by the Examiner UNITED STATES PATENTS 5/44 Parker 174-102 6/59 Brandes et al. 174-102 FOREIGN PATENTS 9/31 Great Britain. 7/40 Germany.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5,175,990 March 16, 1965 Robert P. Lamons It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 14, for "clearly" read clear line 41, for "use" read such column 7, line 17, before "component" insert higher-density column 9, line 1, after "cab1es"insert a period; column 12, line 62, after "regions," insert and line 70, strike out "and"; column 13, line 28, for "co-axial" read coaxial Signed and sealed this 27th day of July 1965.

(SEAL) Attest:

ERNEST W. SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3, 173,990 March 16 1965 Robert P. Lamons It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 14, for "clearly read clear line 41, for "use" read such column 7, line 17, before "component" insert higher-density column 9, line 1,

after "cables"insert a period; column 12, line 62, after regions," insert and line 70, strike out "and"; column 13, line 28, for "co-axial" read coaxial Signed and sealed this 27th day of July 1965.

(SEAL) Altest:

ERNEST W. SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. IN A COAXIAL CABLE COMPRISING AN INNER CONDUCTOR A GENERALLY TUBULAR OUTER CONDUCTOR COAXIALLY SURROUNDING THE INER CONDUCTOR IN SAPCED RELATIN, AND A FOAM DIELECTRIC RIGIDLY SECURED TO THE INNER CONDUCTOR IN THE SPACE BETWEEN THE CONDUCTOR,S THE IMPROVED CONSTRUCTION CHARACTERIZED BY (A) LONGITUDINALLY SPACED COUPLING REGIONS BETWEEN THE DIELECTRIC AND THE INER SURFACE OF THE OUTER CONDUCTOR, (B) THE LENGTH OF THE OUTER CONDUCTOR BETWEEN SAID COUPLING REGIONS BEING GREATER THAN THE LONGITUDINAL DISTANCE THEREBETWEEN, (C) THE COUPLING BETWEEN THE DIELECTRIC AND THE INNER SURFACE OF THE OUTER CONDUCTOR FOR RELATIVE MOTION IN THE LONGITUDINAL DIRECTON BEING SUBSTANTIALLY SMALLER BETWEEN THE COUPLING REGIONS THAN AT THE COUPLING REGIONS, 